Hi @haggis, can you try viewing Overlay on Chrome? It will give a clearer error message.
As for the cause, I think perhaps your piwik.js file is stored on a different server than your piwik install. For example, if your website is using a piwik.js from http://mysubdomain.mypiwik.org/piwik.js and you're viewing Piwik from http://www.mypiwik.org. Or possibly you might be using different protocols (ie, http to get piwik.js and https to view Piwik). The root of the problem is a protocol/domain mismatch in how your site gets the piwik.js file and how you view Piwik.
thank you. I got it running now! The problem was/is that I accessed Piwik via “example.com/piwik”. But Piwik itself tried to load the website within overlay via “www.example.com”. So the problem was in fact a domain mismatch and when I go to “www.example.com/piwik” instead, everything is fine.